Sulfuric acid alkylation process

ABSTRACT

A process for the sulfuric acid catalyzed alkylation of C3-C5 olefins with isobutane is disclosed wherein the requisite mixing is accomplished by the use of eductors within the reactor. In addition the alkylate product is subjected to primary and secondary coalescers for removal of entrained sulfuric acid. The vaporized unreacted C4&#39;s are recovered as liquid by absorption and desorption in an absorber oil.

This application is a continuation of application Ser. No. 11/400,845,which was filed on Apr. 10, 2006 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a reactor/mixer design and process for analkylation process wherein C₃, C₄ and C₅ olefins are reacted withisobutane to produce alkylate in the presence of sulfuric acid catalyst.In one embodiment the invention more particularly relates to a processand apparatus wherein an eductor is used in lieu of mechanical mixer tomix the sulfuric acid and hydrocarbons. In another embodiment moreparticularly the invention relates to a process wherein new the C₃/C₄vapors from the auto refrigeration may be used in lieu of refrigerationcompression. Finally in a third embodiment successive coalescing devicesare used to remove the acid from the hydrocarbon to ppm level.

2. Related Information

The conventional sulfuric acid catalyzed alkylation process has beenpracticed in the past using mechanical propeller mixers which requirecomplex seals. The internal mixer and seals are subjected to a hostileenvironment (strong sulfuric acid which is used as catalyst for theprocess) which is demanding. To provide the mixing and tight emulsionrequired for the process considerable energy is necessary for eachcontactor/reactor. The impellers/seals used require considerablemaintenance. To facilitate the mixing conventional processes are carriedout at temperatures so as to keep the viscosity low, and thesetemperatures are not optimum temperatures to make high octane productwith low acid consumption. The operating expense of the conventionalprocess together with complicated mixing device which has been known torequire higher maintenance can be reduced.

The alkylate product in the refinery economics of gasoline, normally hasvery good return on investment based on the conventional process, butstill the units require high investment, maintenance and liquid wastedisposal. The hydrofluoric acid (HF) alkylation does not requirerefrigeration and acid regeneration so it is marginally better in thatrespect, but due to the high corrosive nature of the HF acid, exoticmaterials of construction are required. The process is also much morehazardous due to the HF acid, and is not readily acceptableenvironmentally.

Mixing devices for mixing the hydrocarbons and acids have been variouslydisclosed in U.S. Pat. Nos. 5,443,799; 3,696,168; 3,133,128; 4,000,212;3,758,613 and 5,220,094. All of these patents disclose mixing deviceswith spray nozzles/spargers/venturi which are all low efficiency mixingequipment options and in most cases have been applied to HF Alkylationwhich have better reaction kinetics than sulfuric acid alkylation. Theart listed in the above referenced patents does not meet thehydrodynamic requirements of an eductor with divergent portion after thenozzle. The previous art is a mixture of spray nozzles/spargers andventuri devices that produce mild mixing and are not suitable forsulfuric acid alkylation high efficiency mixing requirements. Spraynozzles and spargers do mix the liquid but they doe not provide the highefficiency as provided by an eductor. As noted, most of these earlierpatents have been for HF Alkylation, though some are broad in sense butas these are do not provide the same efficiency and hydrodynamics, theycannot be accept as equivalent to eductors due to major hydrodynamicsdifferences.

The vapor from this flash drum can be sent to an absorption column or toa compressor intermediate stage, or if the flash is done at higherpressure it can be condensed separately and recycled to the reactor witholefin feed and isobutane

Due to the side reactions taking place in the process due to theimpurities, acid soluble oils (ASO) which are conjunct polymers, areproduced which reduce the acid concentration and fresh acid make up isneeded to overcome this loss. The acid soluble oil is sent to the acidregeneration unit. The cost of acid regeneration for low acidconsumption unit is about 20% of the operating cost of the alkylationunit.

SUMMARY OF THE INVENTION

An improvement in the process is herein suggested which provides aneductor in lieu of the mechanical mixers. The major improvementsdisclosed in the present invention are (1) the use of a mixing eductordevice, (2) vapor absorption and (3) acid/hydrocarbon separation bycoalescers. Additionally a new method of recovering the C₃/C₄ vaporsfrom the auto refrigeration is utilized introduced for instant process.This saves capital cost by eliminating the need for a refrigerationcompressor since absorption and desorption equipment is less costly thancompressors.

The present invention is particularly useful in converting existing HFalkylation units to sulfuric acid alkylation units since with thepresent invention absorption can be used instead of adding expensivecompression refrigeration equipment.

A coalescing system is provided to obtain lower than 1 ppm level ofsulfur in hydrocarbon product by changing flow regimes, improving thedesign of coalescers and also operating the coalescing under betterconditions i.e. higher temperatures than previously used.

The acid free hydrocarbon is sent to conventional alkylate recoverysection after heat integration. The heat integration system, whichconstitutes a part of one embodiment of the present invention, wherehydrocarbons are heated to flash of C₄'s hydrocarbon from alkylatebetween the coalescing stages, so that one can get better coalescing athigher temperature before the second or successive stages of coalescingand separation. The location of this flash drum provides an efficientacid separation at slightly elevated temperature of 40 to 300 F and heatintegration so as to reduce the quantity of absorption or load on thecompressor, if used.

The vapor from this flash drum may be sent to or to compressorintermediate stage, or if the flash is done at higher pressure it can becondensed separately and recycled to the reactor with olefin feed andisobutane recycle after cooling with the cold alkylate. The vapor fromthis flash drum can be sent absorption or to compressor intermediatestage or if the flash is done at higher pressure than can be condensedseparately and recycled to the reactor with olefin feed and isobutanerecycle after cooling with the cold alkylate. Alkylate is one of mostdesirable gasoline component as it is free of sulfur, aromatics andolefins. Refiners are always looking at improving this alkylationprocess so as to reduce the acid consumption and utilities together withmaintenance costs.

The present invention operates at lower temperature which reduces theacid consumption by 40%, reducing the cost of acid regeneration. Apartfrom the lower acid consumption, major cost benefits of process, beingunique low cost efficient mixing device, auto refrigeration vaporabsorption system and enhanced acid/hydrocarbon separation equipment,which reduces the capital and operating cost of the unit.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified diagram of a reactor according to two separateembodiments of the invention.

FIG. 2 is a more detailed diagram of a reactor according to oneembodiment of the invention.

FIG. 3 is a flow diagram of the acid/hydrocarbon separation processincluded in the subject invention.

FIG. 4 is a block flow diagram of a sulfuric acid alkylation processutilizing one embodiment of the invention.

FIG. 5 is a flow diagram of one option for the C₄ vapor recovery in thepresent invention.

FIG. 6 is a flow diagram of a second option for the C₄ vapor recovery inthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As disclosed herein there is described a process wherein C₃, C₄ and C₅mixed olefins and isobutane stream are mixed vigorously in the presenceof Sulfuric acid catalyst with novel eductor) device. Internal acidpiping is provided to the ejector for the motive fluid. The alkylate,which is produced at a low temperature has a better quality than theconventional process. Essentially the process works under similar acidconcentration as conventional process, e.g., in the range 89 to 95% butpreferably around 90 to 92% to provide the best quality product and lowacid consumption, but at lower temperature than conventional process atabout 20 to 50 F but preferably at 25 to 27 F essentially underisothermal conditions. The low temperature is obtained by autorefrigeration by flashing the C₄ hydrocarbons. Acid to olefin molarratio is kept around 45 to 180, preferably in the range of 45 to 60, toprovide the desired alkylate reaction and quality. An isobutane toolefin molar ratio of 6 to 15 is desired. These ratio's are veryimportant and are controlled to provide the optimum conditions to loweroperating expense. To provide the good mixing all the hydrocarbonstreams are collected and are sent to the eductor where the acid, actingas the motive fluid, mixes all the streams with acid catalyst andreactants to provide good selectivity, and to minimize undesirablereactions.

The disclosed methods and apparatuses can be understood by referring tothe attached figures, which are described in detail herein. It should beunderstood that the pipelines are being designated when streams arebeing identified and that streams are intended, if not stated, whenmaterials are mentioned. Moreover, flow control valves,temperature/pressure regulating devices, pumps, compressors, exchangers,drums and the like are understood as installed and operating inconventional relationships to the major equipment items which are shownin the Figures and discussed hereinafter with reference to the processof the this invention. All of these valves, devices, pumps, compressorand exchangers and the like, are included in the term auxiliaryequipment. It is within the ability of one of the ordinary skill in theart to implement such auxiliary equipment, as needed in view of thepresent disclosure

FIGS. 1 and 2 disclose an embodiment which is part of the reactor andeductor system in the process. The reactor 1 and eductor 2 are the mainpart of this system. The acid settles in the bottom of reactor, which isrecycled by the pump back through line 8, where as the Olefin mixedstream (which includes all the C4 streams recovered and the olefin feed)is fed through line 6 to eductor to provide vigorous mixing of the acidand hydrocarbon. Packing such as intalox, saddles/raschig rings orsimilar, and demister pads, are provided above the eductor, in settlingzone are to separate the hydrocarbon from acid with coalescing andreducing the velocity through packing. As the reactor operates in therange of 1 to 10 psig there is auto refrigeration due to evaporation ofC₃/C₄ and some C₅ hydrocarbons, which are taken out by the line 3 fromthe Top. The reaction products together with some entrained acid andother components are taken for processing to other equipment by line 7as shown in the FIG. 1.

In this embodiment of the invention, the eductor provides adequate andhigh efficiency mixing so as to have the reaction completed as it comesout of the eductor's divergent section. The vapor is generated due toauto refrigeration, and heat of reaction, which is taken as vapor streamfrom the top of the reactor. The reactor effluent reaction products have20-45% alkylate, rest being isobutene/n-butane that is taken as a sidedraw with entrained acid which is separated down stream. As noted above,the reactor is operated at about 1 to 10 psig pressure and 25 to 27 Ftemperature.

FIG. 3 illustrates the treatment of stream 7, which is the reactoreffluent and contains alkylate, C₄ hydrocarbons and entrained acid. Thealkylate and C₄ hydrocarbons are separated from the acid utilizing themultiple coalescers 11 and 14. These coalescers may be the York type orequivalent to meet the product separation requirements. The finishingcoalescer can be kept at the optimum temperature to have betterseparation of acid and hydrocarbon in the range of 80 to 300 F.Recovered acid through line 9 is recycled to the Reactor item 1 afterseparation in first coalescer 11. Line 10 takes the raw mix alkylateproduct through heat exchangers and is flashed to remove some of C₄'s inflash drum 12 with the C₄ stream is taken for recovery via line 13. Thebottoms from the flash drum 12 is sent to finishing coalescer 14 whereall the residual acid is removed. The recovered acid is recycled to thereactor 1 while the raw mix alkylate stream is sent to conventionalrecovery system where alkylate is separated from Isobutane and n-butane.

FIG. 4 provides a block flow diagram of the whole process. Acid strengthis kept at about 90 to 92% by adding acid make up through line 24. Inthe process some adjunct polymers (ASO) are formed, which are taken outthrough the line 25 for acid regeneration. As shown C₄ vapor from line 3is sent to existing compressor in revamp or the new absorption system ofthe present invention. Mixed olefin feed is mixed with all the other C₄streams from the unit before sending it to the reactor. The reactoreffluent from coalescer 14 is sent for alkylate recovery through line20. The normal butane is separated and sent to offsite via line 21,while isobutane is recycled through line 22 to the reactor. Part of theC₄ auto refrigeration vapor after recovery is sent for lights/propaneremoval in depropanizer 19, and is sent off site via line 26.

This embodiment uses reactor effluent cold energy to condense thede-isobutanizer overhead and, or compressor discharge, after cooling thetotal hydrocarbon stream is recycled back to the reactor. This art hasalready been described in the U.S. Pat. No. 4,130,593 of 1978.

The embodiment shown in FIGS. 5 and 6 teaches the absorption of C₄'sproduced from the auto refrigeration in the reactor and taken from thereactor via line 3 in heavy naphtha, special solvent or alkylateproduct. In this embodiment the C₄ streams pressure is increased to 15psig or higher through line 33 by using ejector where the isobutane inline 32 acts as the motive fluids for the absorption liquid in line 31so as to make the absorption possible. In the next column, through line35 the absorbed liquid stream is sent for recovery. The C₄'s arerecovered through line 30 from the heavy naphtha/solvent or alkylate(whichever is used for absorption of C₄ vapors) and the C₄ stream isrecycled back to the reactor. The vent through line 36 is used tocontrol the absorption column. The second column can be reboiled withlow pressure steam and will be a cheaper alternate to the compressoroption and can also be used when revamping any unit where the compressoris a bottleneck. Likewise it may be used for the conversion of HF unitsto sulfuric acid units.

The back end of the unit is a simple de-isobutanizer as one columnsystem or two-column system to separate the isobutane and n-butane fromAlkylate. The straight chain olefins provide the slightly higher octanealkylate from the sulfuric acid alkylation compared to branch chainolefins, and the acid consumption for branched chain olefins is higheras well. So straight chain olefins C₃, C₄ and C₅ are the preferredolefin species.

Reactions

The reaction of olefins with isobutane produces trimethyl pentanes (TMP)which are the desired reaction products. The C₃ olefins provide more ofdimethyl pentane (DMP) in the alkylate and C₅ olefins provide more of C₉alkylate component giving lower octane product compared to C₄ olefins.The straight chain olefins provide the slightly higher octane alkylatefrom the sulfuric acid alkylation compared to branch chain olefins, andthe acid consumption for branched chain olefins is higher as well. Sostraight chain olefins C₃, C₄ and C₅ are the preferred Olefin species.In the Sulfuric Acid alkylation process, olefins and isobutane arereacted in the presence of sulfuric acid catalyst at 20 to 60 F to formessentially TMP and some other byproducts like di-methyl hexanes (DMH)and DMP are formed.

Alkylation ChemistryButylenes+Isobutane→TMPPropylene+Isobutane→DMPIt should be noted that even though in C₄ alkylation TMP has highconcentration over 60 to 70% in the alkylate product, but othercomponents are e.g. DMH and DMP, which are formed in the alkylationreaction.The side reactions can be postulated as follows:Polymerizationolefin+olefin→Polymer C₆, C₈ and C₁₂ etcCrackingIn the reaction larger compounds are made which crack to make smallercompounds:C₁₁H₂₄→C₆H₁₂+C₅H₁₂Hydrogen Transfer ReactionsThis reaction takes place by transferring hydrogen to olefin to makeparaffin. Essentially Isopentane is made in the reaction.2C₄H₁₀+C₅H₁₀→C₈H₁₈+C₅H₁₂Esterification ReactionSulfuric acid reacts with olefins to form small amount of di-butylsulfate, which is unstable at high temperature and is removed in thecoalescer so as to produce good quality product.DisproportionationThis takes place by rearrangement of the hydrocarbons to form differentmolecules from a larger hydrocarbon molecule.2C₁₀H₂₂→C₈H₁₈+C₁₂H₂₆The Relative Reaction Rates of Olefins Influence the Product Quality andRelative Conversion Rates can be Postulated as Follows:n-butene>i-butene>isopentenes>n-pentenes>propylene

From the above one can deduce that one needs lower space velocity forpentenes and even lower for propylene compared to butylenes.

The mixing is done in the eductor and in the reactor where predominantlyTMP mixture, and 20 to 35% of other DMH, DMP and heavy compounds e.g.nonanes are produced.

As it has already been illustrated by prior art U.S. Pat. No. 5,095,168that when working at lower temperatures, around 10 to 50 F, preferablyat 25 to 28 F, the selectivity to TMP is better providing better octaneand also lower acid consumption. The present invention is beingpracticed at the most desired temperature conditions, at isothermalconditions, which provide even better results for selectivity.

EXAMPLE

Feed, wt % Propane/Propylene 0.6 Isobutane 15.5 Isobutylene 14.72-butene 38.5 1-Butene 14.6 N-butane 15.8 C5's 0.3 Total 100.0

The above feed is used for to produced alkylate and the simulatedresults are very good. The Road Octane was observed to be 94.5 to 95.0with research octane clear to be 95.5 to 96.0 and motor octane Clear tobe 93.5 to 94.0.

1. A process for the sulfuric acid catalyzed alkylation of C3-C5 olefinswith isobutane comprising the steps of: (a) feeding sulfuric acid to aneductor in a reactor; (b) feeding a hydrocarbon stream containing C3-C5olefins and isobutane to said eductor wherein the sulfuric acid acts asthe motive fluid to intimately mix the sulfuric acid with thehydrocarbon stream; (c) reacting a portion of the C₃-C₅ olefins with aportion of the isobutane to produce a product stream containing ahydrocarbon portion comprising alkylate product, unreacted C₄'s, andentrained sulfuric acid; (d) separating the hydrocarbon portion of theproduct stream from the sulfuric acid within the reactor by contact witha first demister pad; (e) removing the hydrocarbon stream from thereactor for further processing; and removing the sulfuric acid from thereactor for recycle to the feed.
 2. The process according to claim 1wherein the hydrocarbon portion is further processed in a coelescer toremove sulfuric acid.
 3. The process according to claim 1 wherein thepressure of the C₄'s is increased by the use of an ejector and the C4'sare absorbed in an absorber fluid comprising heavy naphtha.
 4. A processfor the sulfuric acid catalyzed alkylation of C3-C5 olefins withisobutane comprising the steps of: (a) feeding sulfuric acid to aneductor in a reactor; (b) feeding a hydrocarbon stream containing C3-C5olefins and isobutane to said eductor wherein the sulfuric acid acts asthe motive fluid to intimately mix the sulfuric acid with thehydrocarbon stream; (b) reacting a portion of the C₃-C₅ olefins with aportion of the isobutane to produce a product stream containing ahydrocarbon portion comprising alkylate product, unreacted C₄'s, andentrained sulfuric acid; (c) separating the hydrocarbon portion of theproduct stream from the sulfuric acid within the reactor by contact witha first coelescer; (d) subjecting hydrocarbon from step (c) to a secondcoelescer to remove further sulfuric acid; (e) separating the unreactedC₄'s from the alkylate product; (f) increasing the pressure of theunreacted C4's in an ejector and subjecting the unreacted C4's toabsorption in an absorber fluid; (g) removing the unreacted C4's fromthe absorber fluid as a liquid; (h) returning the liquid C4's to thefeed stream of step (b) (i) recovering the acid from the coelescers ofsteps (c) and (d): (j) returning the recovered acid from step (i) to theacid stream of step (a).
 5. The process according to claim 1 wherein thepressure of the C₄'s is increased by the use of an ejector and the C4'sare absorbed in an absorber fluid comprising alkylate.